Mathry 3 is built into the external west face of the churchyard wall at Church of the Holy Martyrs, Mathry (NPRN 412501), just south of Mathry 2 (NPRN 423412). It was moved to its present location from Tegidreg Farm, a possible early medieval chapel/cemetery site (NPRN 423416), some 1.2km south of the church) around 1902.

The stone is an unshaped slab or pillar of quartz-rich grey sandstone. Dimansions are given as 97.5cm height x 43cm < 53.5cm width x diameter not known. Only one face is visible. It is incised with a linear, equal-arm ring-cross within a ring, identical to that on Mathry 2. The carving is thought to date to the 7th-9th century.

Sources include:
Edwards, N. 2007, Corpus of Early Medieval Inscribed Stones and Stone Sculpture in Wales

N Vousden, 11 October 2018